-say you had an Epic Rap Battle on their car with your friends.
-say you practiced tap dancing on their car
-Say you thought their car was sexy and you did things to it
-Say you arranged a riot on their car
-Say you took their car out for a spin and hot boxed it
-Say you street raced with their car
-Say you loosen the screws on their muffler
-Say you played your clarinet solo on their car

When you find someone who claims that there is damage tell the person that you never dinged their car and that you only left the note to teach your son about dishonest lowlifes in the world. Then lecture the person about being dishonest and trying to scam you.